# ProHeart Injection for Dogs: How It Works and Dosing ProHeart injection for dogs is a long-acting heartworm preventive given by your veterinarian as a shot. It is not a vaccine, so it does not teach your dog's immune system to fight heartworms. Instead, it puts a medicine called moxidectin into your dog's body that kills heartworm larvae before they can grow into adult worms. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - ProHeart is a preventive injection, not a vaccine. It does not create immunity to heartworm disease. - The active ingredient is moxidectin, a drug that stays in the body for months and kills heartworm larvae at a specific stage [1]. - Your veterinarian decides the dose based on your dog's body weight, not on age or breed. - A negative heartworm test is required before the first injection in most dogs. - The shot is given in the clinic by a veterinarian or trained technician, usually in the back of the neck or between the shoulder blades. ## What Is ProHeart and What Is It Used For? ProHeart is the brand name for an injectable heartworm preventive for dogs. The active drug is moxidectin. Moxidectin belongs to a class of medicines called macrocyclic lactones. These drugs are widely used to prevent heartworm disease in dogs [1]. Veterinarians use ProHeart to prevent heartworm disease caused by the parasite Dirofilaria immitis. Heartworm disease is serious. It can cause life-threatening damage to the heart, lungs, and blood vessels if left untreated [1]. Many owners search for a "proheart vaccine for dogs." That wording is common, but it is not accurate. A vaccine trains the immune system to recognize and fight a specific germ. ProHeart does not do that. It works as a drug that stays in the bloodstream and kills the parasite directly. The correct term is a preventive injection or a heartworm preventive shot. ProHeart comes in two forms. One is given once a month. The other is given once every six months. Your veterinarian will choose which form fits your dog's lifestyle and your ability to keep up with monthly preventives. ## How Does the ProHeart Shot for Dogs Work? Heartworm disease starts when a mosquito bites an infected dog and picks up baby heartworms called microfilariae. Those microfilariae develop inside the mosquito into infective larvae. When that mosquito bites your dog, it deposits the larvae into your dog's skin. The larvae then travel through the body and mature into adult worms over several months. Adult worms live in the heart and pulmonary arteries. They can grow up to a foot long and cause severe damage. Moxidectin kills the larvae before they become adults. In laboratory studies, dogs were inoculated with 50 infective larvae and then given moxidectin at a minimum effective dose of 0.02 mg/kg. The treatment prevented adult heartworm infection when evaluated about 150 days after inoculation [1]. The timing matters. Moxidectin works against the larval stage. It does not kill adult worms. That is why your dog needs a negative heartworm test before starting ProHeart. If adult worms are already present, the injection will not treat them. Giving a preventive to a heartworm-positive dog can also cause problems if the dog has microfilariae in the blood. ProHeart does not stop mosquitoes from biting your dog. It does not repel mosquitoes. It works inside your dog's body after the bite happens. ## ProHeart Injection for Dogs Dosage: How the Dose Is Decided The ProHeart dose for dogs is not something you calculate at home. Your veterinarian calculates it based on your dog's exact body weight and the specific product being used. ### Who Decides the Dose Your veterinarian decides the dose. The dose depends on your dog's weight in kilograms and the concentration of the product. A small dog gets a smaller volume. A large dog gets a larger volume. The dose is not based on breed, age, or coat type. Your dog will be weighed at the clinic before the injection. Do not estimate the weight yourself. A few pounds can change the dose. ### How the Injection Is Given The injection is given under the skin, usually in the loose skin between the shoulder blades or in the back of the neck. It takes only a few seconds. Most dogs do not react much to the needle. If your dog needs injections at home for another condition, the technique is similar. You can read more about giving injections in our guide on [how to give your dog a shot at home](/knowledge/veterinary-medicine/pet-medication-guide/give-dog-injection-at-home-insulin). ### What Happens at the Vet Visit A typical ProHeart visit includes: - A heartworm test, if your dog is not already on a consistent preventive or if it is the first time. - A weight check. - A brief physical exam. - The injection itself. - A discussion about when the next dose is due. The six-month injection is given twice a year. The monthly injection is given once a month. Your veterinarian will set a schedule. ### What to Do About a Missed Dose If you miss a dose, call your veterinarian right away. Do not wait until the next scheduled visit. The timing of the next injection depends on how late you are and which product your dog receives. Your veterinarian will tell you whether to give the next dose early or whether a heartworm test is needed first. Never give a second injection yourself to make up for a missed one. ProHeart is a prescription product given by a veterinarian. ## Side Effects of ProHeart Injection for Dogs Most dogs tolerate ProHeart well. Side effects can happen, and some are more serious than others. | Side effect | How common or serious | What to do | |---|---|---| | Soreness or swelling at the injection site | Common, mild | Watch for a few days. Call if swelling grows or your dog seems painful. | | Vomiting or diarrhea | Uncommon, usually mild | Withhold food for a few hours. Call your vet if it continues past 24 hours. | | Lethargy or decreased appetite | Uncommon, usually mild | Rest and monitor. Call if it lasts more than a day or two. | | Hives, facial swelling, or itching | Rare, serious | This is an allergic reaction. Call your vet or an emergency clinic right away. | | Difficulty breathing, collapse, or seizures | Very rare, emergency | Go to an emergency veterinary clinic immediately. | Allergic reactions can happen with any injection. They usually appear within minutes to hours after the shot. That is one reason many clinics ask you to wait a short time before leaving. If your dog has a history of reactions to injections or to moxidectin, tell your veterinarian before the shot. ## Who Should Not Take ProHeart and What Interacts With It ProHeart is not right for every dog. Your veterinarian will review your dog's health history before giving the injection. Dogs who should not receive ProHeart include: - Dogs who have not had a negative heartworm test, unless your veterinarian decides otherwise based on your dog's history. - Dogs with a known allergy to moxidectin or similar drugs. - Dogs who are sick, weak, or underweight. - Dogs with certain liver or kidney problems. - Puppies below the minimum age listed on the product label. Tell your veterinarian about every medicine, supplement, and preventive your dog takes. This includes flea and tick products, dewormers, and any other heartworm preventive. Some products contain the same class of drug, and stacking them can increase the risk of side effects. If your dog takes prednisone or another steroid for a health condition, your veterinarian will factor that into the decision. You can learn more about steroid use in our article on [prednisone for dogs](/knowledge/veterinary-medicine/calculators-clinical-tools/prednisone-for-dogs-steroid-dosing-tapering-and-side-effects). If your dog has diabetes and receives insulin, tell your veterinarian. Our guide on [NPH insulin dosing for dogs](/knowledge/veterinary-medicine/calculators-clinical-tools/nph-insulin-dogs-dose-canine-diabetes-twice-daily-protocol) explains how insulin is managed, but it does not replace your vet's advice. ## Overdose and When to Call for Help An overdose of moxidectin can cause serious neurologic signs. These include stumbling, tremors, dilated pupils, drooling, and seizures. The risk is higher in dogs with a genetic sensitivity to certain drugs, in very small dogs, and in dogs who receive multiple products containing the same drug. If you think your dog received too much ProHeart or swallowed another moxidectin product, call your veterinarian or a pet poison control hotline immediately. Do not wait for symptoms to appear. Two resources you can call: - ASPCA Animal Poison Control Center: (888) 426-4435 - Pet Poison Helpline: (855) 764-7661 Have your dog's weight, the product name, and the amount given ready when you call. ## Limitations and When to Contact a Veterinarian ProHeart prevents heartworm disease, but it does not treat an existing infection. It also does not protect against other parasites unless the product also contains other drugs. Your dog still needs flea, tick, and intestinal parasite prevention based on your veterinarian's advice. Contact your veterinarian the same day if: - Your dog vomits repeatedly or has diarrhea that does not stop. - Your dog seems very tired or will not eat. - The injection site is swollen, hot, or painful. - You missed a dose and are not sure what to do next. Go to an emergency veterinary clinic now if: - Your dog has trouble breathing. - Your dog's face or muzzle swells. - Your dog collapses or has a seizure. - Your dog has severe tremors or cannot stand. Call for a routine visit if: - Your dog is due for the next injection. - You are switching from a monthly pill to the injection, or the other way around. - You have questions about which preventive is best for your dog. Every dog is different. Your veterinarian needs to see your dog to make the right call for your situation. ## Frequently Asked Questions ### Is ProHeart a vaccine for dogs? No. ProHeart is a preventive injection, not a vaccine. It contains moxidectin, a drug that kills heartworm larvae in the body. A vaccine would work by teaching the immune system to fight the parasite, and ProHeart does not work that way. ### How often does my dog need the ProHeart shot? It depends on which form your veterinarian uses. One form is given once a month, and another is given once every six months. Your veterinarian will set the schedule and remind you when the next dose is due. ### Can my dog get the ProHeart shot without a heartworm test? In most cases, no. Your veterinarian will want a negative heartworm test before starting the injection, especially if your dog has not been on a consistent preventive. Giving a preventive to a dog with adult heartworms does not treat the infection and can cause complications. ### What should I watch for after the injection? Watch the injection site for swelling or soreness. Watch your dog for vomiting, diarrhea, tiredness, or loss of appetite. Serious allergic reactions like hives, facial swelling, or trouble breathing are rare but need emergency care right away. ### Can I give ProHeart at home? No. ProHeart is a prescription injection given by a veterinarian or trained veterinary staff. The dose depends on your dog's exact weight and the product used, and your dog needs a heartworm test and exam before the shot. If your dog needs other types of injections at home, talk to your veterinarian about training and safety. ## Related Articles - [How to Give Your Dog a Shot at Home: Insulin and Other Injections](/knowledge/veterinary-medicine/pet-medication-guide/give-dog-injection-at-home-insulin) - [Prednisone for Dogs: Steroid Dosing Tapering and Side Effects](/knowledge/veterinary-medicine/calculators-clinical-tools/prednisone-for-dogs-steroid-dosing-tapering-and-side-effects) - [NPH Insulin Dogs Dose: Canine Diabetes Twice Daily Protocol](/knowledge/veterinary-medicine/calculators-clinical-tools/nph-insulin-dogs-dose-canine-diabetes-twice-daily-protocol) - [Dog Allergy Shots: How They Work and What to Expect](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/dog-allergy-shots) - [How to Give Your Dog a Subcutaneous Fluid Injection at Home](/knowledge/veterinary-medicine/senior-and-chronic-care/give-dog-subcutaneous-fluids-home) - [Pimobendan Vetmedin Cardiac Dose: Weight-Based CHF Dog Dosing Chart](/knowledge/veterinary-medicine/calculators-clinical-tools/pimobendan-vetmedin-cardiac-dose-weight-based-chf-dog-dosing-chart) ## References 1. [Efficacy of a novel chewable tablet (Credelio Quattro™) containing lotilaner, moxidectin, praziquantel, and pyrantel for the prevention of heartworm disease (Dirofilaria immitis) in dogs](https://pubmed.ncbi.nlm.nih.gov/41943128/). Parasites & vectors, 2026. ## Further Reading - [American Veterinary Medical Association: First Aid Tips for Pet Owners](https://www.avma.org/resources-tools/pet-owners/emergencycare/first-aid-tips-pet-owners) - [Merck Veterinary Manual: Dog Owners](https://www.merckvetmanual.com/dog-owners) - [Cornell Richard P. Riney Canine Health Center: Canine Health Information](https://www.vet.cornell.edu/departments-centers-and-institutes/riney-canine-health-center/canine-health-information) - [American Veterinary Medical Association: Pet Care](https://www.avma.org/resources-tools/pet-owners/petcare)